Earth observation satellites that photograph the surface of the earth by using a synthetic aperture radar (SAR) or optical lenses are operated on an orbit around the earth, in addition to communication satellites that communicate with communication terminals on the earth. When the number of orbiting satellites being operated is small, an earth station keeps following, that is, tracking an orbiting satellite that passes through a visible range thereof, thereby being able to establish a feeder link with the orbiting satellite. However, when the number of communication satellites, that is, the number of orbiting satellites being operated is increased with an increase of communication demand, a plurality of orbiting satellites may pass through the visible range of the earth station, and this makes it difficult to use the operating method of the earth station described above. From the viewpoints of the cost and construction site, it is not preferable to construct the same number of earth stations as the number of operated orbiting satellites, and thus it is desired to efficiently track orbiting satellites with less earth stations.
Patent Literature 1 discloses a technique of, in a satellite communication system constituted by a plurality of quasi-zenith satellites, deriving the time required for switching a satellite to be tracked from the coordinates of the respective satellites, and selecting the satellite to be tracked such that the downtime of an earth station is minimized.